The present invention relates to coverings which can be applied to the surface of articles of elastomeric material, in particular tires, in order to decorate them and, in particular, coverings which can be applied after the vulcanizing of said articles.
The need to emphasize markings or ornamental designs on the surface of articles of prevulcanized elastomeric materials by means of a covering, such as a paint, has existed for a long time. By way of example, U.S. Pat. No. 2,088,561 proposes applying to the surface of a tire a liquid organic composition comprising pigments and rubber and U.S. Pat. No. 3,263,990 discloses the use of epoxy systems. All of these coverings have the major drawback of rapidly becoming detached from the surface of the covered tires.
In order to solve this problem, U.S. Pat. No. 4,670,496 proposes a liquid paint combining an organic solvent, a mixture of unvulcanized diene elastomers and accelerators, and pigments. This paint employs the free sulfur of the elastomeric mixtures on which it is applied in order to effect a cold vulcanization and thus create a durable bond.
Although it solved the problem of the durability of the bonding, this paint, however, has several drawbacks: the anti-ozone waxes and the antioxidant chemical agents can migrate through this paint and therefore the aesthetic appearance may be impaired due to the efflorescing of the waxes or the coloring due to the antioxidant chemical agents; the use of an organic solvent in a substantial amount raises problems of industrial hygiene; and finally the mixing of the pigments into the paint requires a large amount of pigments which are frequently very costly. The invention discloses a decorative laminate which substantially improves the preceding problems without degradation which is prejudicial to its adherence to the surface of the elastomeric article covered.
The mechanical behavior of the non-cross-linked polymers varies, as a function of the temperature, from a zone which is vitreous at low temperatures in which the behavior is vitreous, that is to say rigid and brittle, to a zone of fluid flow at high temperatures. Between these two zones there is a so-called "rubber plateau" in which the behavior is rubbery, that is to say close to that of an elastomer provided that the molecular weight of the polymer is sufficient so that there have been interlacings (see: "Viscoelastic Properties of Polymers," John D. Ferry, 3rd Ed., John Wiley & Sons, 1980, in particular Chapters 10, 12 and 13).
The "glass transition temperature" of a polymer ("Tg") is the temperature at which the mechanical behavior of the polymer evolves from this rigid, brittle vitreous behavior to this rubbery behavior. This glass transition temperature is an essential characteristic of polymers.
The glass transition temperatures are generally determined by differential enthalpy analysis (see Introduction to Thermal Analysis: techniques and applications--Michael E. Brown, Ed., Chapman and Hall, New York, 1988). This technique, better known by its acronym DSC (differential scanning calorimetry) consists of determining the variations in specific heat of a sample the temperature of which is increased. It makes it possible to show transitions or reactions which are accompanied by liberation of energy (exothermal) or the absorption of energy (endothermal). The glass transition is an endothermal transition.